Work has already been done to try to evaluate petrophysical properties from drill cuttings. The acoustic properties as regards S and P waves (shear and compressional waves) have been measured for example. Various parameters have also been studied, such as the hardness and the deformation of the rock fragments, or their porosity and permeability.
According to a first known method intended for permeability measurement, the rock fragment is first coated with resin. A thin slice is cut out of the coated rock and placed in a measuring cell. It comprises means for injecting a fluid under pressure at a controlled flow rate and means for measuring the pressure drop created by the sample. Since the resin is impermeable, the absolute permeability is deduced from Darcy's equation by taking account of the real surface area occupied by the rock fragments.
This method is for example described by:    Santarelli F. J., et al.; <<Formation evaluation from logging on cuttings>>, SPERE, June 1998, or    Marsala A. F., et al.; <<Transient Method Implemented under Unsteady State Conditions for Low and Very Low Permeability Measurements on Cuttings>>, SPE/ISRM No.47202, Trondheim, 8–10 July 1998.
This type of measurement can only be obtained in the laboratory after long fragment conditioning operations.
Another well-known method is based on a NMR (Nuclear Magnetic Resonance) measurement which is directly carried out on the cuttings after prior washing followed by brine saturation. This type of measurement gives a porosity value that can be directly used. The permeability K is determined by means of correlations of the same nature as those used within the context of NMR logging.
An illustration of this method can be found in the following document:    Nigh E., et al.; P-K™: Wellsite Determination of Porosity and Permeability Using Drilling Cuttings, CWLS Journal, Vol.13, No.1, December 1984.
Patent application EP-1,167,948 and patent applications FR-02/02,242 and 03/00,429 describe several systems for evaluating physical parameters such as the absolute permeability of porous rocks from a zone of an underground reservoir, from rock fragments taken in this zone, such as rock fragments carried along by the drilling mud. It comprises a stage of immersing fragments contained in a containment chamber in a viscous fluid and of placing the chamber containing the fragments under pressure according to several implementation modes so as to compress the gas trapped in the pores of the rock.
According to an implementation mode, a fluid is injected at a pressure that increases with time, up to a predetermined pressure threshold, so as to compress the gas trapped in the pores of the rock. This injection stage is followed by a relaxation stage where injection is stopped. The pressure variation during these two successive stages is recorded.
According to another implementation mode, the chamber containing the fragments is communicated with a vessel containing a viscous fluid at a predetermined pressure. The communication stage being short, a fast pressure increase in the chamber and compression of the gas trapped in the pores of the rock are obtained, followed by a relaxation period after isolation of the chamber, and the pressure evolution in the chamber during the two periods is measured.
According to another implementation mode, the chamber containing the fragments is communicated with a vessel containing a viscous fluid at a predetermined constant pressure so as to cause a fast and prolonged pressure increase in the chamber and compression of the gas trapped in the pores of the rock, and the evolution of the volume of fluid injected is measured as a function of time.
Whatever the operating mode, the evolution of the pressure or of the volume of fluid injected having been modelled from initial values selected for the physical parameters of the fragments, the computer adjusts them iteratively so as to obtain the best possible match between the modelled pressure or volume curve and the pressure or volume curve really measured.